Optical pyrometers are well known in the art, having found extensive use in applications in severe environments or where temperature magnitudes prohibit the use of conventional contact pyrometric techniques. These devices calculate the temperature of a target from the radiant energy provided therefrom. An algorithm is used to determine the temperature of the surface by measuring total radiation in a given wavelength interval or by looking at the distribution of optical energy as a function of wavelength. The higher the temperature of the source, the greater the proportion of optical energy in the shorter wavelengths.
Optical pyrometers have been developed to measure the temperature of turbine blade surfaces even in an operating jet engine whose environment necessarily includes the combustor flame fireball. Pyrometers in jet engine applications provide a temperature indication to the electronic engine control system which sets the engine operating point. In jet engine development, pyrometers are used to validate designs and provide measurement of critical parameters such as combustor exit temperature. To accurately measure the turbine blade temperature, the optical pyrometer must be capable of correcting the measurements to eliminate the effect of high levels of radiation from the combustor flame which obscures the radiation emitted by the turbine blade.
Multiple spectral area optical pyrometers have been developed in order to differentiate between reflected and emitted radiation received from a target turbine blade and compensate for the error in the observed temperature that the reflected radiation introduces. In U.S. Pat. No. 4,222,663; incorporated herein by reference, Gebhart, et al discloses a dual band (two color) optical pyrometer which comprises two separate pyrometers. Each pyrometer sees a different but overlapping component of the total spectral range of the light or radiation from the turbine blade.
The pyrometers are sensitive at different wavelength bands and will be affected differently by the energy from the turbine blade surface. When the light (radiation) from the fireball is reflected off the blade, the pyrometer set to detect the shorter wavelength band is more responsive to the additional reflected energy, and is output signal increases in greater proportion than does that of the longer wavelength pyrometer. Therefore, an increase or decrease in the amount of the reflected radiation or, at high reflection conditions, to the temperature of the combustion flame will result in a proportionally higher or lower value of temperature indicated in the short wavelength pyrometer.
For each pyrometer an algorithm calculates the temperature of the turbine blade from the light it receives. The reflected energy of the much hotter combustion flame causes each of the two pyrometers to yield different temperature values, both higher than the true blade temperature. An additional temperature correction algorithm receives each channel temperature and determines the magnitude of the temperature error. The temperature correction signal is a function of the difference between the two pyrometer temperatures, which results from the spectral range of each pyrometer, the fireball equivalent blackbody temperature and the fraction of reflected radiation present in each pyrometer signal.
U.S. Pat. No. 4,797,006 to Masom, incorporated herein by reference, discloses a pyrometer system for use with a gas turbine engine that provides output signals to a processor indicative of the temperature of rotating blades in a gas turbine engine. The processor includes a synchronization unit and a gate controlled by the synchronization unit after monitoring the output of the pyrometer to identify which parts of the output signal rise from the radiation from the blades and which arise from combustion chamber. The synchronization unit is set in synchronism with the blade signals and the gate is controlled to pass signals from the blade to interrupt signals arising from the combustion chamber. The output of the processor is thereby indicative of the blade temperature.
Multiple-spectral band pyrometers have been developed to measure and correct for the reflected radiation. U.S. Pat. No. 4,708,474, also incorporated herein by reference, describes a dual spectral area pyrometer (DSAP) which is based on the principal that two blackbody calibrated pyrometers sensitive to different wavelengths will not respond equally when subjected to a radiative input containing surface emitted radiation plus reflected radiation from a another source at a much higher temperature. The '474 pyrometer includes an optical guide for receiving from the target an optical beam that has an emitted component from the target and a reflected component from a fireball. A detection module receives and divides the target optical beam into first and second optical beams and provides electrical signal equivalents thereof. The second optical beam is selected to have a spectral width that is a portion of the target beam spectral width. Also included in the '474 pyrometer is a signal processor that provides for receiving the first and second signals as well as an energy ratio signal. The signal processor provides reflection corrected energy signals from a difference between the first signal and the product of the energy ratio signal and the second signal.
It is well known that pyrometer systems measuring turbine blade temperatures in jet engines are subject to radiation from three different sources. The pyrometer simultaneously collects reflected radiation from surrounding surfaces, radiation from combustion (flame) in the pyrometer field of view and radiation emitted by the blade. The radiation emitted by the blade is the component required to measure the blade temperature. Consequently, the contribution by reflection in the flame must be subtracted from the total radiation collected by the pyrometer. The '006 and '474 Patents describe systems with methods to subtract these two components subject to certain restraints and conditions in the input signal.
The three radiative components behave differently. At a steady state operating point, only the blade emitted component remains constant. The flame and reflected signal componentry vary between the blades and for the same blade from one engine revolution to the next. The flame component varies randomly in intensity and location as the pyrometer field of view sweeps over the blade surface. Depending on the jet engine power setting, the intensity reaches a level which can saturate the signal detectors and processors for a large portion of the turbine revolution. At the same time, the intensity of the reflected component varies along the blade surface. This variation is driven by geometry and view factors. The intensity of the reflected radiation depends on the luminosity emitted by the combustor and it varies with time.
However, these multiple spectral area pyrometers do not correct for random signal saturation caused by the thermal emission of combustion in the field of view commonly referred to as "flame". The spectral characteristics of the combustor must be known. In jet engines the pyrometer signal can be affected by flame for a significant portion of a rotor revolution. Under such conditions turbine blade temperatures cannot be monitored.